Method and apparatus for operating a field-effect transistor (FET) pair

ABSTRACT

A power switch, and a method, for use with a power switch having a field-effect transistor (FET) including source, drain and gate terminals. The power switch includes a first field-effect transistor (FET) having a first drain coupled to the drain terminal, a first source coupled to the source terminal, and a first gate; and, a second FET having a second drain coupled to the drain terminal, a second source coupled to the source terminal, and a second gate. The second FET has a gate length (L G ) that is greater than or less than an L G  of the first FET and has a length of a drain (L D ) that is greater than or less than an L D  of the first FET. The power switch further includes a control circuit coupled to the gate terminal, the first gate, and the second gate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of and claims priority under 35 U.S.C. §120 to commonly-owned, U.S. application Ser. No. 10/327,586, filed on Dec. 20, 2002, now U.S. Pat. No. 6,703,888, which is a continuation of U.S. application Ser. No. 10/172,484, filed on Jun. 13, 2002, now U.S. Pat. No. 6,529,056, which is a continuation of U.S. application Ser. No. 09/853,356, filed on May 11, 2001, now U.S. Pat. No. 6,433,614, which is a continuation of U.S. application Ser. No. 09/798,008, filed on Mar. 2, 2001, now abandoned, the entire contents of which are hereby incorporated by reference.

BACKGROUND

This disclosure relates to transistor switches, and more particularly to metal oxide semiconductor field-effect transistor (MOSFET) switches.

In power management ICs having a monolithically integrated MOSFET power train, the on-chip field-effect transistor (FET) not only accounts for most of the power dissipation, but also can consume a significant amount of silicon area, and very often is the major concern regarding the long-term reliability of the chip.

FIG. 1 shows a cross-sectional view of a conventional asymmetric high-voltage NMOS transistor, compatible with standard CMOS processes. Although implementations are described with reference to an asymmetrical NMOS device, other types of MOSFET devices are applicable.

An N+ source region 104 is formed within a P-type substrate 102. Also formed in P-type substrate 102 is an N-type deep drain (NDD) region 106 that includes an N+ drain implant 108, and an N-type lightly doped drain (LDD) implant 112. Also, formed upon P-type substrate 102 is a gate 114.

Two important dimensions in the device structure shown in FIG. 1 are the length L_(G) of gate 114 and the length of the drain L_(D) (e.g., the spacing between the N+ drain implant 108 and gate 114). Design rules for these two dimensions can be set to meet two specifications: punch-through breakdown voltage, and hot-carrier lifetime.

Quite often, the hot-carrier lifetime specification (also referred to as the reliability specification), instead of the punch-through breakdown voltage specification, determines the design rule which dictates the minimum allowed dimensions of L_(G) and L_(D). In other words, in the applications where hot-carrier degradation is not of concern, a more aggressive design rule can be used to design a transistor such as that shown in FIG. 1 with smaller dimensions of L_(G) and L_(D) while still meeting the same punch-through breakdown voltage specification. A FET structure with smaller dimensions of L_(G) or L_(D) is generally preferred because such smaller dimensions not only reduce the overall chip area, but may also reduce the on-resistance and the junction capacitance of the FET, thus improving overall system efficiency.

Hot-carrier injection (HCI) typically occurs at the overlapping period between the transitions of the gate voltage and drain voltage of the FET, with the injection peaking when the gate voltage is approximately one half of the drain voltage. As a result, the typical inverter application turns out to be a stressful operation for a FET in terms of hot-carrier degradation. HCI is discussed in greater detail in W. Weber, C. Werner and A. V. Schwerin, “Lifetimes and substrate current in static and dynamic hot-carrier degradation”, IEDM 86, pp 390–393, 1986.

FIG. 2 shows a conceptual time t versus voltage v plot of voltage waveforms for a conventional N-FET during switching transitions of a typical inverter mode operation. During the turn-on transition, the drain voltage V_(D) goes low and the gate voltage V_(G) goes high. During the turn-off transition, V_(D) goes high and V_(G) goes low. The area between times t₁ and t₂ and t₃ and t₄ shows the transition period during which strong hot-carrier injection generally occurs. Hot-carrier degradation may result in threshold voltage shift and transconductance degradation of an N-FET. Due to hot-carrier degradation concerns, a conventional design of a FET switch typically involves trade-offs between electrical performance, such as on-resistance, and reliability performance, such as hot-carrier lifetime. In general, making a conventional device more resilient to hot-carrier degradation involves increasing one or both of L_(G) and L_(D), while improving electrical performance (and minimizing device area) involves minimizing L_(G) and L_(D).

SUMMARY

In general, in one aspect, the specification describes a power switch having source, drain and gate terminals. The power switch further includes a first field-effect transistor (FET) having a first drain coupled to the drain terminal, a first source coupled to the source terminal, and a first gate; and, a second FET having a second drain coupled to the drain terminal, a second source coupled to the source terminal, and a second gate. The second FET has a gate length (L_(G)) that is greater than or less than an L_(G) of the first FET and has a length of a drain (L_(D)) that is greater than or less than an L_(D) of the first FET. The power switch further includes a control circuit coupled to the gate terminal, the first gate, and the second gate.

Particular implementations can include one or more of the following features. The control circuit can be configured to turn on the second FET before turning on the first FET. The control circuit can be coupled to the drain terminal. The control circuit can be configured to impose a fixed delay between turning on the first and second FETs. The control circuit can be configured to delay turning on the first FET until the voltage between the drain and source terminals falls below a predetermined voltage. The control circuit can be configured to turn off the second FET after turning off the first FET. The control circuit can be configured to impose a fixed delay between turning off the first and second FETs. The first FET can be designed for electrical performance superior to that of the second FET. The first FET can be an n-type CMOS FET or an LDMOS transistor. The second FET can be designed for reliability performance superior to that of the first FET. The first and second FETs, or the first and second FETs and the control circuit, can be implemented as a single monolithic device. The second FET can have an L_(G) greater than an L_(G) of the first FET and an L_(D) greater than an L_(D) of the first FET.

The power switch can further include a current sensing circuit configured to disable the first FET when the load current at the drain terminal is below a predetermined threshold current. The current sensing circuit can be configured to switch the second FET when the first FET is disabled. The first FET can have an area less than the area of the second FET. The second FET can have an L_(G) smaller than an L_(G) of the first FET and an L_(D) smaller than an L_(D) of the first FET. L_(D) can be a measurement that represents spacing between an N+ drain implant and a gate of a given FET. The second FET can be more resilient to hot-carrier degradation than the first FET. The second FET can have a wider safe operating area (SOA) than the first FET. The second FET can have a higher avalanche energy rating than the first FET.

In general, in another aspect, the specification describes a method of operating a power switch having a source terminal and a drain terminal. The method includes turning on a first field-effect transistor (FET) that has a first drain coupled to the drain terminal, a first source coupled to the source terminal, and a first gate; turning on a second FET while the first FET is on, the second FET having a second drain coupled to the drain terminal, a second source coupled to the source terminal, and a second gate, the second FET having a gate length (L_(G)) that is greater than or less than an L_(G) of the first FET and having a length of a drain (L_(D)) that is greater than or less than an L_(D) of the first FET; turning off the second FET while the first FET is on; and turning off the first FET.

Particular implementations can include one or more of the following features. Turning on and off the second FET and first FET can include directing signals from a control circuit to the first and second gates. The method can further include any of the following steps: receiving a signal in the control circuit from the drain terminal; delaying turning on the second FET for a predetermined period of time after turning on the first FET; determining a voltage between the drain and source terminals; delaying turning on the second FET until the voltage between the drain and source terminals falls below a predetermined voltage; delaying turning off the first FET for a predetermined period of time after turning off the second FET. The method can also further include any of the following steps: determining a load current at the drain terminal; disabling the second FET when the load current at the drain terminal is below a predetermined threshold current; and switching the first FET when the second FET is disabled.

Advantages that can be seen in implementations of the invention include one or more of the following. Implementations of the invention provide cost reduction, efficiency improvement and reliability enhancement in switching applications. A helper FET can only accounts for a small percentage of a total FET switch size, allowing designers to cut an overall area of a FET power switch while improving overall switching efficiency. Such an approach successfully overcomes the tradeoff between electrical performance and reliability performances of conventional power MOSFET switches.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a cross-sectional view of a conventional asymmetric high-voltage NMOS transistor.

FIG. 2 is a time t versus voltage v plot of voltage waveforms for a conventional N-FET during the turn-on transition of a typical inverter mode operation.

FIG. 3 is a block diagram of a FET switch according to one implementation.

FIG. 4 depicts a circuit for use in an FET control circuit according to one implementation.

FIG. 5 shows a timing diagram for three of the voltage waveforms for a switch according to one implementation.

FIG. 6 depicts a circuit for use in an FET control circuit according to another implementation.

FIG. 7 depicts a circuit for use in an FET control circuit according to still another implementation.

FIG. 8 depicts a circuit for use in an FET control circuit according to yet another implementation.

FIG. 9 depicts a circuit for use in an FET control circuit according to another implementation.

FIG. 10 is a graph illustrating current-voltage trajectories during switching transition of a FET switch according to one implementation.

FIG. 11 shows a cross-sectional view of a lateral double diffused MOSFET (LDMOS) transistor.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 3 is a block diagram of a power FET switch 302 according to one implementation. FET switch 302 includes a main FET F_(M), a helper FET F_(H), and a control circuit 304. Control circuit 304 controls the timing of the operation of the main FET F_(M) and the helper FET F_(H). In one implementation, the main FET F_(M) is designed for superior electrical performance, while the helper FET F_(H) is designed for superior reliability performance. The helper FET F_(H) can control the drain voltage of the main FET F_(M) during the switching transition of the main FET F_(M).

The drain D_(M) of the main FET F_(M) is coupled to the drain D_(H) of the helper FET F_(H) to form the drain D of FET switch 302. The source S_(M) of the main FET F_(M) is coupled to the source S_(H) of the helper FET F_(H) to form the source S of FET switch 302. Control circuit 304 receives control signals applied to the gate G of FET switch 302. In some implementations, control circuit 304 also receives control signals applied to the drain D of FET switch 302.

In one implementation, control circuit 304 controls the timing of the gate signal applied to terminal G_(M) of the main FET F_(M), and the gate signal applied to terminal G_(H) of the helper FET F_(H) such that the main FET F_(M) operates at a stress-free biasing condition under any switching scenario.

FIG. 4 depicts a circuit 400 for use in control circuit 304 according to one implementation. The inputs of a NAND gate 402 are coupled to terminals G and G_(H). The inputs of a NOR gate 404 are coupled to terminals G and G_(M). The output of NAND gate 402 is coupled to the input of an inverter 406. The output of inverter 406 is coupled to terminal G_(M). The output of NOR gate 404 is coupled to the input of an inverter 408. The output of inverter 408 is coupled to terminal G_(H).

FIG. 5 shows a timing diagram for three of the voltage waveforms for FET switch 302 according to one implementation. V_(D) is the voltage appearing at terminal D. V_(G)(F_(H)) is the voltage applied to terminal G_(H) by control circuit 304. V_(G)(F_(M)) is the voltage applied to terminal G_(M) by control circuit 304. V_(G) is the voltage appearing at terminal G. V_(G) is substantially similar to V_(G)(F_(H)) during the turn-on transition, and is substantially similar to V_(G)(F_(M)) during the turn-off transition. Therefore, for clarity, V_(G) is not shown.

A turn-on transition is shown from time t₁ to time t₅. At t₁, V_(G) begins to rise. FET switch 302 responds by turning on the helper FET F_(H). Control circuit 304 turns on the helper FET F_(H) by asserting a high voltage V_(G)(F_(H)) at terminal G_(H). Consequently, V_(G)(F_(H)) begins to rise, and V_(D) begins to drop. At time t₂, the helper FET F_(H) is on, so V_(G)(F_(H)) is high and V_(D) is clamped at a clamped voltage V_(C).

At time t₃, FET switch 302 turns on the main FET F_(M). The main FET F_(M) can be turned on before the helper FET F_(H) is completely on, as long as the drain voltage has dropped to a level at which HCI is no longer a concern. Control circuit 304 turns on the main FET F_(M) by asserting a high voltage V_(G)(F_(M)) at terminal G_(M). At time t₄, V_(D) begins to fall from V_(C) to VON. At time t₅, the main FET F_(M) is on, so V_(G)(F_(M)) is high and V_(D) has reached VON. As can be seen, control circuit 304 delays the main FET F_(M) transition for a fixed delay time T_(D1)=t₃−t₁. Delay time T_(D1) can be increased by adding buffer stages to control circuit 304.

A turn-off transition is shown from time t₆ to time t₁₀. At time t₆, V_(G) begins to fall. FET switch 302 responds by turning off the main FET F_(M). Control circuit 304 turns off the main FET F_(M) by asserting a low voltage V_(G)(F_(M)) at terminal G_(M). Consequently, V_(G)(F_(M)) begins to fall.

Depending on load conditions, V_(D) may rise. In one implementation, the portion of the V_(D) curve shown from time t₆ to time t₁₀ represents the maximum voltage V_(D) is allowed to reach. FET switch 302 keeps V_(D) at or below this maximum.

At time t₇, V_(D) is restricted to at or below V_(C). At time t₈, the main FET F_(M) is off.

At time t₉, FET switch 302 turns off the helper FET F_(H). The helper FET F_(H) can be turned off before the main FET F_(M) is completely off, as long as the drain voltage remains at a level at which HCI is no longer a concern. Control circuit 304 turns off the helper FET F_(H) by asserting a low voltage V_(G)(F_(H)) at terminal G_(H). Consequently, V_(D) is no longer clamped at or below V_(C), and so V_(D) may rise. At time t₁₀, the helper FET F_(H) is off, so V_(G)(F_(H)) is low and V_(D) can be high. As can be seen, control circuit 304 delays the helper FET F_(H) transition for a fixed delay time T_(D2)=t₉−t₆. Delay time T_(D2) can be increased by adding buffer stages to control circuit 304.

As can be seen from FIG. 5, in the static state, the main FET F_(M) and helper FET F_(H) operate in parallel. However, their operations differ during the switching transient period. The helper FET F_(H) turns on before the main FET F_(M) turns on, thereby lowering the voltage across the main FET F_(M) during its turn-on transition. The helper FET F_(H) also turns off after the main FET F_(M) turns off, thereby limiting the voltage across the main FET F_(M) during its turn-off transition. Therefore, the main FET F_(M) experiences no HCI stress.

In one implementation, the main FET F_(M) is free of the reliability design constraint, and can be designed for optimal electrical performance. For example, the main FET F_(M) can be designed using more aggressive design rules than a conventional FET not only to reduce the silicon area, but also to improve the efficiency of FET switch 302. In one implementation, the helper FET F_(H) sustains all the HCI stress, and is designed for robust and reliable performance. The helper FET F_(H) can be designed using conventional, or even more conservative, design rules to achieve this performance.

In one implementation, the magnitude of the benefits of FET switch 302 is a function of the relative size (or channel width) of the main FET F_(M) and the helper FET F_(H). Only the main FET F_(M) portion contributes in terms of area saving and efficiency improvement. Thus, according to one implementation, the smaller the main FET F_(M) is relative to the helper FET F_(H), the greater the benefit.

In one implementation, the overall size of FET switch 302, including both the main FET F_(M) and the helper FET F_(H), is a function of an on-state voltage drop (VON) requirement. In switching applications, VON typically is a very low voltage level. The helper FET F_(H) alone is on to clamp the drain of the main FET F_(M) at a voltage lower than the blocking voltage that FET switch 302 will otherwise see during conventional switching. Blocking voltage is the voltage that a switch sustains in the off-state. The relative size of the helper FET F_(H) to the size of the helper FET F_(H) and the main FET F_(M) combined is inversely proportional to the ratio of clamped voltage (V_(C)) to VON.

Hot-carrier injection quickly subsides as clamped voltage V_(C) decreases from the blocking voltage. Therefore, clamped voltage V_(C) can be much higher than zero while still being low enough to protect the main FET F_(M) from hot-carrier stress. In one implementation, clamped voltage V_(C) in FET switch 302 has a much greater voltage range than conventional switching modes.

FIG. 6 depicts a circuit 600 for use in control circuit 304 according to another implementation. The inputs of a NAND gate 602 are coupled to terminals G and G_(H). The input of an inverter 604 is coupled to terminal G. The input of an inverter 606 is coupled to the output of NAND gate 602. The output of inverter 606 is coupled to terminal G_(M). The output of inverter 604 is coupled to the input of an inverter 608. The output of inverter 608 is coupled to terminal G_(H). As can be seen, circuit 600 delays the main FET F_(M) transition for a fixed delay time T_(D1)=t₃−t₁. Circuit 600 implements the timing of FIG. 5 only for the turn-on transition of FET switch 302.

FIG. 7 depicts a circuit 700 for use in control circuit 304 according to still another implementation. The input of an inverter 702 is coupled to terminal G. The inputs of a NOR gate 704 are coupled to terminals G and G_(M). The input of an inverter 706 is coupled to the output of inverter 702. The output of inverter 706 is coupled to terminal G_(M). The output of NOR gate 704 is coupled to the input of an inverter 708. The output of inverter 708 is coupled to terminal G_(H). Circuit 700 implements the timing of FIG. 5 only for the turn-off transition of FET switch 302.

FIG. 8 depicts a circuit 800 for use in control circuit 304 according to yet another implementation. The input of an inverter 802 is coupled to terminal G. The inputs of a NOR gate 804 are coupled to terminals G and G_(M). The inputs of a NOR gate 806 are coupled to terminal D and the output of inverter 802. The input of an inverter 808 is coupled to the output of NOR gate 804. The output of inverter 808 is coupled to terminal G_(H). Circuit 800 implements the timing of FIG. 5 only for the turn-on transition of FET switch 302.

Circuit 800 implements a variable delay T_(DV)=t₃−t₁ during the turn-on transition of FET switch 302, when the effects of HCI are more severe than during the turn-off transition. The turn-on of the main FET F_(M) is delayed until the drain voltage V_(D) falls below a predetermined voltage. In this implementation, clamped voltage V_(C) is designed to be within the range of an effective logic “low.” Circuit 800 implements a fixed delay T_(D2)=t₉−t₆ during the turn-off transition of FET switch 302.

In one implementation control circuit 304 includes a current sensing circuit. When the load current falls below a predetermined threshold, control circuit 304 shuts off main FET F_(M). Switching is then accomplished by helper FET F_(H) alone.

For a given size for FET switch 302, the conduction loss of FET switch 302 decreases with decreases in DC load current. When FET switch 302 is operated at a light load current condition, the power losses incurred by charging up the gate capacitance of FET switch 302 (including both the main FET F_(M) and the helper FET F_(H)) may dominate the overall loss of FET switch 302. Therefore, the conduction loss of FET switch 302 becomes negligible, and the overall efficiency of FET switch 302 improves due to the dramatic reduction of gate capacitance and charging loss associated with the gate capacitance. In this situation, it is useful to disable the main FET F_(M) and use the helper FET F_(H) only.

FIG. 9 depicts a circuit 900 for use in control circuit 304 according to this implementation. The inputs of a NAND gate 902 are coupled to terminals G and G_(H), and to the output of a current sensing circuit 910. The inputs of a NOR gate 904 are coupled to terminals G and G_(M). The output of NAND gate 902 is coupled to the input of an inverter 906. The output of inverter 906 is coupled to terminal G_(M). The output of NOR gate 904 is coupled to the input of an inverter 908. The output of inverter 908 is coupled to terminal G_(H).

Current sensing circuit 910 outputs a logic high level when the drain current (that is, the current at drain D) is greater than a predetermined threshold current (indicating a normal load). Current sensing circuit 910 outputs a logic low level when the drain current is less than the predetermined threshold current (indicating a light load). Such current sensing circuits are well-known in the relevant arts. Circuit 900 thus operates in a manner similar to circuit 400 in FIG. 4 under normal loads. However, under light loads, circuit 900 keeps the main FET F_(M) shut off at all times, while the helper FET F_(H) is free to carry out the function of FET switch 302.

Safe Operating Area

In addition to improving efficiency improvement and reliability enhancement in terms of HCI, implementations and techniques (including timing requirements) described above are also useful for improving other aspects of a power switch, for example, to widen the safe operating area (SOA) of a power switch. HCI and SOA concerns are generally related to the same MOSFET device characteristic—i.e., the generation of substrate impact ionization current when the MOSFET device is subjected to stress under both high voltage and high current.

FIG. 10 shows a graph 1000 illustrating current-voltage (I-V) trajectories 1002, 1004, of a main FET F_(M) and a helper FET F_(H), respectively, during switching transitions according to one implementation. In this implementation, the helper FET F_(H) is designed to achieve a wider SOA boundary 1006 relative to an SOA boundary 1008 of the main FET F_(M). Thus, the main FET F_(M) can be designed having a superior electrical performance (such as having a low on-resistance) relative to the helper FET F_(H).

By timing turn-on and turn-off sequences between the main FET F_(M) and the helper FET F_(H), the helper FET F_(H) can control the I-V trajectory (or locus) 1002 of the main FET F_(M) so that the main FET F_(M) turns on and off according to zero-voltage switching (ZVS), as shown in FIG. 10. ZVS is less demanding on the SOA of the device. By relieving the SOA constraints of the main FET F_(M), the main FET F_(M) can, therefore, be optimized to have a better electrical performance than the helper FET F_(H).

Avalanche Energy Capability

In one implementation, FET switch 302 includes a main FET F_(M) designed for superior electrical performance, and a helper FET F_(H) designed with a higher ruggedness (or avalanche energy rating). Device ruggedness can be defined by the capacity of a device to sustain an avalanche current during an unclamped inductive load switching event. The helper FET F_(H) can be designed with a higher avalanche energy rating, but with a lower avalanche breakdown voltage (BV) relative to the main FET F_(M). In such an implementation, only the helper FET F_(H) will reach an avalanche breakdown state during a switching transient. The helper FET can protect the main FET by clamping a voltage spike (associated with avalanche breakdown), and the high avalanche energy rating of the helper FET can ensure ruggedness of FET switch 302.

With respect to timing between gate signals of the main FET F_(M) and the helper FET F_(H), there can be no specific timing requirement. For example, both the main FET F_(M) and the helper FET F_(H) can be driven simultaneously, or according to timing requirements as discussed above in connection with HCI degradation and SOA.

LDMOS Transistor

As discussed above, implementations can include other types of MOSFET devices. One example of another type of MOSFET device is a lateral double diffused MOSFET (LDMOS) transistor.

FIG. 11 shows a cross-sectional view of an LDMOS transistor 1100. LDMOS transistor 1100 can be fabricated on a high voltage n-type well (HV n-well) 1102 implanted in a p-type substrate 1104. An HV n-well is typically used for high voltage LDMOS transistors—e.g., greater than 30V. HV n-well 1102 can have a retrograded vertical doping profile. LDMOS transistor 1100 includes a drain region 1106, a source region 1108, and a gate 1110. Drain region 1106 includes an n-doped n+ region 1112 and an n-doped shallow drain (N-LD) 1114. Source region 1108 includes an n-doped n+ region 1116, a p-doped p+ region 1118, and a p-doped P-body 1120.

Referring to FIG. 3, in one implementation, FET switch 302 includes a main FET F_(M) implemented using a CMOS N-FET and a helper FET F_(H) implemented using a more robust LDMOS transistor. An LDMOS transistor is intrinsically more robust than a CMOS N-FET due to LDMOS transistors having a deeper drain characteristic with respect to CMOS N-FETs. In this implementation, the LDMOS transistor can have larger or smaller dimensions of L_(G) and L_(D) relative to the CMOS N-FET, even though the CMOS N-FET is designed having superior electrical performance.

In one implementation, FET switch 302 includes a main FET F_(M) and a helper FET F_(H) each implemented using a LDMOS transistor. The main FET F_(M) can have smaller dimensions of one or all of L_(G), L_(D), and a channel length relative to the helper FET F_(H). The main FET F_(M) can have a shallow or lightly doped p-body implant (to lower a threshold voltage of the main FET F_(M) for a low on-resistance), and the helper FET F_(H) can have a deep or heavily doped p-body implant for better ruggedness. Alternatively, other implants of one LDMOS can have a different doping and/or size relative to the other LDMOS transistor to achieve different operating characteristics.

Implementations can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Apparatus can be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions described above by operating on input data and generating output.

Implementations can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer will include one or more mass storage devices for storing data files; such devices include magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits).

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, FET switch 302 can be implemented monolithically, or as two or more discrete components. The main FET F_(M), the helper FET F_(H), or both can be implemented as a single FET or as many FETs operating together. FET switch 302 can be implemented using N-type MOSFETs or P-type MOSFETs. FET switch 302 can be implemented to affect only the turn-on transition, only the turn-off transition, or both. FET switch 302 can be implemented to drive capacitive, resistive or inductive loads. In addition, the length of a drain L_(D) can be measured from a gate of a FET to any implant region within the drain of the FET, and can also vary depending upon conventions of a particular FET. Accordingly, other implementations are within the scope of the following claims. 

1. A power switch having source, drain and gate terminals, the power switch comprising: a first field-effect transistor (FET) having a first drain coupled to the drain terminal, a first source coupled to the source terminal, and a first gate; a second FET having a second drain coupled to the drain terminal, a second source coupled to the source terminal, and a second gate, the second FET having a gate length (L_(G)) that is greater than or less than an L_(G) of the first FET and having a length of a drain (L_(D)) that is greater than or less than an L_(D) of the first FET; and a control circuit coupled to the gate terminal, and having a first control signal coupled to the first gate and a second control signal coupled to the second gate, the first control signal and the second control signal being separate control signals, and the first control signal having a rising edge with a different timing than a rising edge of the second control signal.
 2. The power switch of claim 1, wherein the control circuit is configured to turn on the second FET before turning on the first FET.
 3. The power switch of claim 1, wherein the control circuit is coupled to the drain terminal.
 4. The power switch of claim 3, wherein the control circuit is configured to impose a fixed delay between turning on the first and second FETs.
 5. The power switch of claim 4, wherein the control circuit is configured to delay turning on the first FET until the voltage between the drain and source terminals falls below a predetermined voltage.
 6. The power switch of claim 1, wherein the control circuit is configured to turn off the second FET after turning off the first FET.
 7. The power switch of claim 6, wherein the control circuit is configured to impose a fixed delay between turning off the first and second FETs.
 8. The power switch of claim 1, wherein the first FET is designed for electrical performance superior to that of the second FET.
 9. The power switch of claim 8, wherein the first FET is an n-type CMOS FET.
 10. The power switch of claim 8, wherein the first FET is an LDMOS transistor.
 11. The power switch of claim 1, wherein the second FET is designed for reliability performance superior to that of the first FET.
 12. The power switch of claim 1, wherein the first and second FETs are implemented as a single monolithic device.
 13. The power switch of claim 1, wherein the first and second FETs and the control circuit are implemented as a single monolithic device.
 14. The power switch of claim 1, wherein the second FET has an L_(G) greater than an L_(G) of the first FET and an L_(D) greater than an L_(D) of the first FET.
 15. The power switch of claim 1, further comprising: a current sensing circuit configured to disable the first FET when the load current at the drain terminal is below a predetermined threshold current.
 16. The power switch of claim 15, wherein the current sensing circuit is configured to switch the second FET when the first FET is disabled.
 17. The power switch of claim 1, wherein the first FET has an area less than the area of the second FET.
 18. The power switch of claim 1, wherein the second FET has an L_(G) smaller than an L_(G) of the first FET and an L_(D) smaller than an L_(D) of the first FET.
 19. The power switch of claim 1, wherein L_(D) is a measurement that represents spacing between an N+ drain implant and a gate of a given FET.
 20. The power switch of claim 1, wherein the second FET is more resilient to hot-carrier degradation than the first FET.
 21. The power switch of claim 1, wherein the second FET has a wider safe operating area (SOA) than the first FET.
 22. The power switch of claim 1, wherein the second FET has a higher avalanche energy rating than the first FET.
 23. The power switch of claim 1, wherein the control circuit includes one or more buffer stages for delaying the timing of the rising edge of the first control signal relative to the timing of the rising edge of the second control signal.
 24. A method of operating a power switch having a source terminal and a drain terminal, comprising: turning on a first field-effect transistor (FET) using a first control signal having a first rising edge, the first FET having a first drain coupled to the drain terminal, a first source coupled to the source terminal, and a first gate; turning on a second FET using a second control signal having a second rising edge while the first FET is on, the second control signal being separate from the first control signal, the second rising edge having a different timing than first rising edge, the second FET having a second drain coupled to the drain terminal, a second source coupled to the source terminal, and a second gate, the second FET having a gate length (L_(G)) that is greater than or less than an L_(G) of the first FET and having a length of a drain (L_(D)) that is greater than or less than an L_(D) of the first FET; turning off the second FET using the second control signal while the first FET is on; and turning off the first FET using the first control signal.
 25. The method of claim 24, wherein the turning on and off the second FET and first FET includes directing signals from a control circuit to the first and second gates.
 26. The method of claim 25, further comprising receiving a signal in the control circuit from the drain terminal.
 27. The method of claim 24, further comprising delaying turning on the second FET for a predetermined period of time after turning on the first FET.
 28. The method of claim 24, further comprising determining a voltage between the drain and source terminals.
 29. The method of claim 28, further comprising delaying turning on the second FET until the voltage between the drain and source terminals falls below a predetermined voltage.
 30. The method of claim 24, further comprising delaying turning off the first FET for a predetermined period of time after turning off the second FET.
 31. The method of claim 24, further comprising determining a load current at the drain terminal.
 32. The method of claim 31, further comprising disabling the second FET when the load current at the drain terminal is below a predetermined threshold current.
 33. The method of claim 21, further comprising switching the first FET when the second FET is disabled.
 34. The method of claim 24, wherein the second FET is designed for electrical performance superior to that of the first FET.
 35. The method of claim 24, wherein the helper FET is designed for reliability performance superior to that of the main FET.
 36. The method of claim 24, wherein the first FET and the second FET are implemented as a single monolithic device.
 37. The method of claim 24, wherein the first FET has an area less than the area of the second FET.
 38. The method of claim 24, wherein L_(D) is a measurement that represents spacing between an N+ drain implant and a gate of a given FET.
 39. The method of claim 24, wherein the second FET is an n-type CMOS FET.
 40. The method of claim 24, wherein the second FET is an LDMOS transistor.
 41. The method of claim 24, further comprising using one or more buffer stages to delay the timing of the rising edge of the second control signal relative to the timing of the rising edge of the first control signal. 